It has been conventionally qualitatively known that carbonitriding treatment is effective to extend rolling contact fatigue life of a rolling bearing (see for example “Rolling Fatigue Characteristics of Carburized or Carbonitrided 1% Cr Steel at Elevated Temperatures” by Hyojiro Kurabe et al., Iron and Steel, vol. 11, (1967), p. 1305 (hereinafter referred to as Non-Patent Literature 1) and Japanese Patent No. 2962817 (hereinafter referred to as Patent Literature 1). Further, in recent years, it has become clear that carbonitriding treatment is also effective in a method for evaluating scratched contact fatigue life of a rolling bearing, which is a more quantitative method, as described in Japanese Patent Laying-Open No. 2009-229288 (hereinafter referred to as Patent Literature 2). For example, it has been found that, if a nitrogen concentration in a ground uppermost surface is not less than 0.1 mass %, the life is extended to the extent that its statistically significant difference from that of a non-nitrided part is reliably recognized.
Accordingly, if it is possible to provide a rolling bearing part which is quality-assured to have a nitrogen concentration of not less than 0.1 mass % in a ground uppermost surface of a carbonitrided part, safety of a rolling bearing is ensured more reliably, and safety of mechanical equipment using the rolling bearing is improved, providing a great social advantage as a result.
At present, however, a nitrogen concentration in steel can be quantified only by a method performed using analysis equipment such as an EPMA (Electron Probe Micro Analyzer) and a GDS (Glow Discharge Spectrometer), and such measurement requires a great number of man-hours. Therefore, if a method using analysis equipment as described above is adopted as an inspection method for quality assurance, the rolling bearing becomes very expensive, which is not practical.
In addition, a method for qualitatively evaluating the degree of nitridation by utilizing the characteristics of nitrogen which has entered steel that “the hardness after high-temperature tempering is higher than that of a non-nitrided portion” is described, for example, in “Effects of Nitrogen Content on Microstructure and Resistance to Softening during Tempering of Carbo-Nitrided Chromium Alloy Steels” by Youichi Watanabe et al., Heat Treatment, vol. 40, (2000), p. 18 (hereinafter referred to as Non-Patent Literature 2), and the like.